valgrind: Add --support-valgrind configure option
When --enable-valgrind is passed, PCRE will annotate some memory regions
as unaddressable, aiding in finding invalid memory accesses when running
under valgrind. This is useful mostly for debugging PCRE itself, and thus
probably should not be enabled in release builds.

pcre32: docs: Document that non-character codepoints are excluded
Document that UTF-8/16/32 validity checks now exclude the non-character
code points, which are U+FDD0..U+FDEF and the last two code points in each
plane, U+??FFFE and U+??FFFF.

pcre32: fullinfo: Add variants of (FIRST|LAST)LITERAL that are 32-bit clean
Since for pcre32 the whole range of the output is already used up
for the character itself, return the special values separately.

pcre32: Add 32-bit library
Create libpcre32 that operates on 32-bit characters (UTF-32).
This turned out to be surprisingly simple after the UTF-16 support
was introduced; mostly just extra ifdefs and adjusting and adding
some tests.

General spring-clean of EBCDIC-related issues in the code, which had decayed
over time. Also the documentation. Added one test that can be run in an ASCII
world to do a little testing of EBCDIC-related things.

(1) Move internal flags out of the options field, to make room.
(2) \r and \n must be explicit to trigger the special CRLF handline exception.
(3) (?J) at the start now sets JCHANGED as well as DUPNAMES.

Create the PrepareRelease script to process the documentation and create the
.generic files for distribution, also to remove trailing spaces. Update a lot
more of the build-time documentation. Arrange for PrepareRelease and its
sub-scripts to be distributed.

1. Move the comment about version numbers from pcre.h.in to configure.ac
because that's where they are now set.
2. Update all the man pages to remove the use of .br and .in because this
causes trouble for some HTML converters. Also standardised the final sections
giving author information and revision date.
3. Update the maintain/132html man page converter to handle .nf/.fi and to barf
at .br/.in.

Applied Bob and Daniel's patches to convert the build system to automake. Added
the maintain directory, containing files that are used for maintenance, but are
not distributed. This is an intermediate step.